Conventionally known is a battery state detection device (battery sensor) that detects the state of a battery. Such battery state detection devices are disclosed in, for example, Patent Documents 1 and 2.
The battery state detection device of this type includes a shunt resistor used for detection of a current, a circuit board, and the like. The battery state detection device may include a casing that houses the shunt resistor and the circuit board.
The casing, for example, has a box-like shape made of plastic. A board storage space for storing the shunt resistor, the circuit board, and the like, is formed in the casing. The casing is configured such that a lid part for closing an open end of the storage space can be mounted to the casing. The lid part is mounted in a state where the shunt resistor, the circuit board, and the like, are housed in the casing, thus achieving in hermetic sealing of the casing.
Particularly in the field of vehicles, it is demanded that a battery state detection device have a high reliability, mainly from the viewpoint of safety. For example, entry of water into the casing of the battery state detection device may cause malfunction or failure because of the water adhering to the circuit board or the like. This is why a high degree of hermetic sealing (waterproofness) is demanded of the casing of the battery state detection device.
In this respect, the conventional battery state detection device has an O-ring arranged between a casing and a lid part, to ensure the hermetic sealing of the casing.